The invention is directed to improvements in circuit breakers and more particularly to a novel and improved method and structure for protecting a thermal trip assembly of a circuit breaker from excessive thermal energy.
In circuit breakers having a thermal trip mechanism, the mechanism is also responsive to a thermal energy responsive element such as a bi-metallic element for tripping the breaker to an open position. For example, a bi-metallic strip deforms so as to activate or trigger a trip mechanism of the breaker in response to a predetermined current/time profile of current flowing through the breaker which reflects the current flowing through the circuit to be protected by the breaker. The mechanism rotates the moveable contact assembly so as to open the current path by moving the moveable contact away from the fixed contact.
The thermal mechanism should be protected from excessive thermal energy to avoid damage to the bi-metallic element and/or other elements of the thermal trip assembly. However, this must be done in such a way as not to interfere with the desired sensing and reactions to current flowing through the breaker by other trip mechanisms including an electromagnetic trip mechanism and a blow back function (described below).
Accordingly, one or more positive temperature coefficient resistance elements have heretofore been added to the current path. These resistances elements have a relatively low resistance at normal ambient operating temperatures and the resistance increases according to a given resistance versus temperature curve or profile, which may be specified in the design of the PTC element and/or material. However, given constraints of space and cost for circuit breakers of this type, it is not generally economically feasible to design, specify and add yet further components to the current path. Moreover, the addition of yet further components such as additional PTC resistance elements, further increases the complexity and expense of fabrication and assembly of the breaker.
Accordingly, the invention provides for one or more pre-existing elements in the breaker current path to be constructed of a suitable positive temperature coefficient material and to be appropriately configured and dimensioned to present a desired PTC profile for increasing resistance in the current path in response to increasing temperature in such a manner as to protect the thermal trip elements of a breaker without compromising operation of other trip mechanisms of the breaker.